Due to the film's length, 1900 was presented in two parts when originally released in many countries, including Italy, East and West Germany, Denmark, Belgium, Norway, Sweden, and Colombia. Other countries, such as the United States, released an edited version of the film.[4]

Born on the day of the death of renowned composer Giuseppe Verdi—27 January 1901—Alfredo Berlinghieri and Olmo Dalcò come from opposite ends of the social spectrum. Alfredo is from a family of landowners led by his populist grandfather (also called Alfredo), while Olmo is an illegitimate peasant. Olmo's grandfather, Leo, is the foreman and peasants' strong man who verbally and spiritually carries out a duel of wits with the elder Alfredo. As Alfredo is somewhat rebellious and despises the falseness of his family, in particular his weak but abusive and cynical father Giovanni, he befriends Olmo, who was raised as a socialist.

The two are friends throughout their childhood, despite the social differences of their families. Olmo enlists with the Italian army in 1917 during World War I and goes off to fight while Alfredo learns how to run his family's large plantation under the guidance of his father. Olmo returns from the war over a year later and his friendship with Alfredo continues. However, Alfredo's father has hired Attila Mellanchini as his foreman. Taken with fascism, Attila eventually incorporates his new belief system in his dealings with the Berlinghieri workers; he treats them cruelly and later cages them in the Berlinghieri compound and accuses them of treason against fascist Italy. Several are killed by Attila himself. As the new padrone (master) of the plantation, Alfredo does little to challenge or halt Attila's actions.

During the late 1920s, the intimacy and lack thereof in their respective relationships with others is highlighted in their love lives. Alfredo marries a gorgeous, demure woman while Olmo marries Anita, who like him shares in the enthusiasm of the cause of workers' rights. Alfredo’s wife, Ada, sinks into alcoholism when confronted with the reality of the emptiness of her relationship with Alfredo. Anita, a strong and independent spirit dies tragically in childbirth, bringing another member into the community. As Olmo takes on his fateful role of leader among the poor farmers and their families, he clashes with Attila.

The power, however, shifts after World War II in 1945, and the ruling class is at the mercy of the jovial and bitter peasants in the agricultural estate. As padrone, Alfredo is captured by a teenage peasant boy carrying a rifle. Attila is also captured when he and his wife, Regina, try to flee the region. Attila is stabbed, non-fatally, several times by women wielding pitchforks and is imprisoned in the Berlinghieri pig sty. He is later executed by the peasants (while they cut off most of Regina's hair), who have discovered that Attila had killed a young boy (ironically, the son of one of the most fervent supporters of fascism as an antidote to socialism) several years prior and had also murdered a wealthy landowner's widow, Mrs. Pioppi (whose husband had been economically ruined by Alfredo), in order to steal her land and home. Years earlier, Attila also had several peasants killed after they threw horse manure at him for "selling" Olmo. Olmo made Attila eat manure and let him go. Olmo then had to leave town to keep from being killed by the fascists. Alfredo fires Attila, when Attila and his blackshirts seek vengeance on Olmo by tearing up Olmo's house.

In the final scenes, set on 28 April 1945, Alfredo is brought before Olmo's workers tribunal to stand trial. Many workers come forth and accuse Alfredo of letting them suffer in squalor while he profited from their labors. Alfredo is sentenced to death, but his execution is prevented after Olmo explains that the padrone is dead, so Alfredo Berlinghieri is alive, suggesting that the social system has been overthrown with the end of the war. As soon as the verdict is reached, however, representatives and soldiers of the new government, which includes the Communist Party, arrive and call on the peasants to turn in their arms. Olmo convinces the peasants to do so, overcoming their skepticism. Alone with Olmo, Alfredo declares, "The padrone is alive."

The original director's cut of the film runs 317 minutes (5 hours, 17 minutes). Alberto Grimaldi, the film's producer, was contractually obligated to deliver a 195-minute (3 hours, 15 minutes) version to Paramount Pictures. Bertolucci originally wanted to release the film in two parts, but Grimaldi refused.

Grimaldi then locked Bertolucci out of the editing room, and assembled a 180-minute cut. Bertolucci, horrified at Grimaldi's cut, decided to compromise.[5] His 247-minute (4 hours, 7 minutes) version was the one initially released in the United States. In 1987, the Bravo channel broadcast the uncut version with dubbed dialogue. Later in 1991, the film was restored to its original length and shown in a limited release. The film has also been shown uncut on Sky Movies and Film 4.

When Bertolucci released his 317-minute version to theaters, the Motion Picture Association of America re-classified the film with an NC-17 rating; the 245-minute American cut, the other version officially available on video in the United States, still retained its R rating. In 2006, Paramount surrendered the NC-17 rating of the uncut version, then released it as unrated on DVD on 5 December 2006. This same uncut version was released on Blu-ray Disc in the US by Olive Films on 15 May 2012 and remains on sale in North America.

The music for the movie was composed by Ennio Morricone, who included several melodies from Verdi operas. One of his own melodies, a cantata, was adopted as the hymn of the Spanish Socialist Workers' Party.[9]